Optical fiber signal modulation may be achieved in a number of ways, including modulation of emitter drive electronics, phase modulation of an arm of an optical fiber interferometer and spoiling a waveguide to create modulated signal attenuation. The latter two approaches are called external modulators as distinct from modulators which vary the strength of a signal emitted from a source. In-fiber gratings have been used to reflect and transmit optical carrier signals in varying proportions. Mechanical strain and thermal expansion have been described as means of varying the proportions of reflected and transmitted light for narrow band optical carrier signals striking an in-fiber grating.
Piezoelectric acoustic transducer material has been applied as a buffer around optical fiber to induce strain in the fiber core, resulting in phase modulation of optical carrier signals guided by the fiber. In prior art, a piezoactive polymer jacket, approximately 120 .mu.m thick was used to produce phase modulation in single mode optical fiber having 80 .mu.m fused silica diameter. Phase modulator performance was demonstrated over a frequency range from 300 Hz to 2.6 MHz. This work was published in 1984: J. Jarzynski:, "Frequency response of a single-mode optical fiber phase modulator utilizing a piezoelectric plastic jacket," J. Appl. Phys, 55(9), 3243, (1984). In the prior art: 1) Determination of piezoelectric electromechanical conversion transducer shell thickness is not described as a trade off between maximum voltage (limited by the product of dielectric strength of the transducer material with shell thickness) against pulse spreading which reduces maximum frequency response of the transducer when shell thickness goes above one quarter of the transducer material acoustic wavelength at the maximum frequency, said maximum frequency being limited by the criterion that acoustic wavelength must be at least equal to the optical waveguide mode field diameter. 2) Acoustic impedance matching among electromechanical conversion transducer shell material, conductive inner and outer sleeve materials and optical waveguide material is not identified as a means of reducing unwanted reflections to the point where acoustic resonance and signal modulation interference from reflections are substantially eliminated. 3) Determination of the maximum attainable magnitude of focusing gain is not described as that obtained when the formula for focusing gain incorporating both square root of the ratio of transducer diameter to minimum acoustic wavelength (set equal to waveguide mode field diameter) and exponential decay of acoustic wave particle velocity with propagation through an absorbing medium has a maximum value versus the independent variable of transducer shell diameter. 4) Shifting the optical frequency spectrum of an in-fiber reflection grating is not described as a means of providing nearly 100% modulation depth of an optical signal at up to 0.5 GHz modulation frequency.
The invention relates to external modulation of optical fiber carrier signals using focused acoustic waves to modify the reflection spectrum of an in-fiber grating.